Scientific computing

(for the rest of us)

Flowcharts

One of the most powerful tool to plan a programming task is to draw a flochart. In simple terms, a flowchart will let you map the different steps that the program will have to follow, and see what is required for each of them. To illustrate, we will use a flowchart not of a program, but of a pancake recipe.

At the beginning, it can be useful to break the task down into coarse steps:

graph LR prepare --> cook cook --> eat

This is not much to look at, but this is a very high-level view of what we want to achieve. It is also easy to zoom in on every task, and try to further break it down into smaller tasks. Let’s do this for the task at left: prepare.

graph LR liquid[liquid ingredients] --> batter solid[solid ingredients] --> batter batter --> cook cook --> eat

“All” we have done so far is to split the prepare step into the preparation of solid and liquid ingredients. To come up with a full flowchart, we need to repeat this process until we are satisfied by the completeness of it:

graph LR liquid[liquid ingredients] --> mix{mix} solid[solid ingredients] --> mix mix --> batter batter --> cook{cook} cook --> eat{eat}

Notice that we have introduced a new shape: the rotated square will be used to note actions, and anything else will be either an output or an input of these actions.

We like the idea of using different shapes to indicate different steps: what goes in, what is the action, are there any artifacts produced or side-effects? This being said, even a very coarse flowchart can help you.

If we repeat the exercise for most of the boxes, we may end up with a flowchart like this. It might seem a little overwhelming at first, but this is a helpful document. It maps out the precise steps and processes in your project, and can help you both to plan what to write, but also to understand what you have written previously.

graph TD flour --> sift{sift} sugar --> mix1{mix} bak[baking powder] --> mix1 salt --> mix1 sift --> mix1 milk --> mix2{mix} eggs --> whisk1{whisk} whisk1 --> mix2 butter --> melt{melt} melt --> mix2 mix1 --> combine{combine} mix2 --> combine combine --> batter griddle[flat griddle] --> heat{heat} batter --> cook{cook} heat --> cook cook --> flip{flip} flip --> cook2{cook} cook2 --> pancake pancake --> eat{eat}

See how a simple task (“make crepes!”) can be broken down into dozens of litle steps, each representing a single well-defined action? Programming is, essentially, that. We are going to need to specificy things as unambiguously as possible, and breaking the big tasks down is going to be immensely helpful.